First, it is necessary to describe the general terrestrial telephone system. This is the "larger telecommunication system" mentioned above.
The terrestrial telephone system comprises the sum of all of the wire, optical, point-to-point microwave and any other means of conducting signals from one point to another via the surface of the earth, together with their associated switches, amplifiers, encoders, decoders, amplifiers and repeaters. On a national level, the network links all access points in a country. Country is linked to country via international trunk lines. Oceans are spanned by submarine cables. The terrestrial network is a transparent carrier where a signal may be introduced from an access point and routed to another, selected access point elsewhere on the surface of the earth. A subscriber can access the terrestrial system via various gateways. For example, the terrestrial system can be accessed from the public switched telephone network, a public switched data network, an integrated switched digital network, a cellular telephone system, or a satellite telephone system. Each gateway provides a plurality of different access points, which, depending upon the gateway, can range from a simple telephone to a complex data or image source. The subscriber's gateway, at the subscriber's request, commands the terrestrial system to route and carry the message from the subscriber to a selected destination. The message comes in through the subscriber's gateway, passes through the terrestrial system, and exits through the selected destination gateway to reach the access point required. Once the message path is established, two-way communication can commence. This is the way the terrestrial system works. Each gateway is connected to another single gateway. The terrestrial system is not adapted, readily, to switch, during the course of a message, between different source gateways or between different destination gateways.
Each gateway, in what is generally understood to be a telephone system, provides a plurality of access points whereby a subscriber may make or receive calls. Each access point is unique in its identify or location and definitely is unique to its particular gateway. In other words, one access point means one gateway.
Now, satellite systems provide a gateway to the terrestrial system via earth stations, capable of establishing two way contact with a satellite or satellites. In turn, the satellites can communicate with user terminals (portable handsets). The user terminals are thus put in contact with the earth stations which can act as a gateway to the terrestrial system.
Sadly, each earth station is a separate gateway. Generally, the earth stations are widely spread apart. The present invention imagines an earth station density of one, two or just a few earth stations on each continent. Thus, in general, each user terminal will see satellites which are visible only to one earth station. This satisfies the terrestrial network requirement "one access point means just one gateway", This situation is not a problem. However, there a circumstances, for example, in the middle of the pacific ocean, in the middle of the Sahara Desert, High atop the Andes, where a user terminal may be able to see a satellite or satellites which, in turn, are visible to more than one earth station. This now breaks the rule "one access point means just one gateway". The user terminal is an access point. Each earth station is a gateway. Now there is the situation "one access point-many gateways". The terrestrial network is not adapted to respond to multiple gateways. Further, the signals to and from the earth station, satellites and user terminals are forever changing. What was a perfectly good satellite link a moment ago is no longer functional because the satellite has moved, the user terminal has been shielded by a building or other structure, and a host of other reasons. However, one of the other user terminal to satellite to earth station links might still work. How can the switch between earth stations be effected without confusing the operation of the terrestrial network? In particular, how can a call be established between a user terminal and the terrestrial system using the intervening medium of plural satellites and plural earth stations? The present invention seeks to provide a system and a method whereby a user terminal may gain access to the terrestrial system through a single earth station gateway even though alternative earth stations may have to be employed for contact with the user terminal.